This invention relates to optical systems in general and more particularly relates to an improved apparatus particularly adapted for use with a microscope system to enable a user to view phase or transparent objects.
The prior art is cognizant of the fact that certain objects cannot be seen with the ordinary microscope with ordinary illumination. Such objects are virtually transparent under such ordinary conditions and are sometimes referred to as phase objects.
For example such an object may occupy a thin, transparent, but inhomogeneous layer consisting, for instance, of an optically denser body embedded in a rarer surrounding medium, with a relatively sharp boundary between them. Both the object and the medium will have relatively equal transmittance but will also possess a difference in phase.
In an ordinary microscope phase differences ensure equality of intensity and therefore the object is indistinguishable from the medium or the object image is indistinguishable from the adjacent image.
Furthermore, since the eye is sensitive to intensity and not to phase, such objects are invisible, even though these objects retard or advance the phase of the light wave passing therethrough.
There exists a number of specialized microscopes which enable a user to view such phase objects. Such devices are referred to in the prior art as phase-contrast microscopes and interference microscopes. A good description of such devices appears in a book entitled "Analytical Cytology" by R. Barer, Chapter 3, published by McGraw-Hill, N.Y. (1965). The phase contrast microscope sometimes referred to as the Zernike phase contrast microscope employs a phase plate. Such a plate has an annular central region which coincides with an annular opening in a source housing, the central region is formed, for example, by the deposition of a ring of a phase altering medium which has in addition, an absorbing medium, serving to give all light arriving at the image through the central region a path differing by one quarter of a wave length from the light that misses the central region. Thus due to this region, the image of the object is either brighter or fainter than that of the surroundings, depending upon the phase difference introduced at the phase plate. In such a system, defracted light passing the central region produces halos around the image.
The interference microscope is also a phase device and operates by also introducing an inhomogenity to cause a phase shift to thereby enable one to distinguish the object from the surrounds. A disadvantage of the interference microscope is the loss of light at the several semi-reflecting surfaces and the need for polarization and its dependence on strain free optical parts and finally the high precision required to construct the optical birefringent elements. Such devices are in wide-spread use in the study of tissue masses and related fields.
Recently a new device which is the subject matter of my co-pending application Ser. No. 476,518 filed on June 5, 1974 and entitled MODULATION CONTRAST MICROSCOPE, has been discovered.
This device operates on the amplitude of the beam of light passing through a phase object. By operating on selected portions of the amplitude of light passing through the Fourier plane of an optical system as existing in a compound microscope, one can render a transparent object's phase gradients visible by converting the phase information to intensity variations at the the real image plane of the microscope. The application describes a unique and inexpensive modulator positioned at the Fourier plane and having different transmitting regions serving to create light amplitude differences which operate to render such phase objects completely visible.